Escherichia coli (E. coli) is a gram-negative, rod-shaped bacterium. Although most strains of E. coli are benign and are found as normal intestinal flora of humans and other animals, some strains are pathogenic and can lead to sometimes-fatal disease. Different strains of pathogenic E. coli differ in their epidemiology, clinical course and potential for causing outbreaks of disease. Passage of disease is generally through the fecal/oral route.
Pathogenicity has been linked to several serotypes, as defined by O and H antigens. Different pathogenic serotypes are associated with different clinical disease courses and have associated with them different levels of concern from the standpoint of public health. Several outbreaks of disease have been tracked to food and water borne sources of pathogenic E. coli. 
One serotype of E. coli in particular, serotype O157:H7, has been associated with several food and water borne outbreaks and is regulated as an adulterant in ground beef by the U.S. Department of Agriculture (USDA) with a zero tolerance standard. This serotype of E. coli is believed to have arisen from an O55:H7 parent strain, which then switched from O55 to O157 upon the transfer into the progenitor O55:H7 genome of the large virulence plasmid pO157, which contained the O157-rfb gene cluster as well as some additional genetic information (see, e.g., Lukas M. Wick, et al., Evolution of Genomic Content in the Stepwise Emergence of Escherichia coli O157:H7, Journal of Bacteriology 187:1783-91 (2005)).
Since E. coli is ubiquitous, and since serotype O157:H7 is highly pathogenic and tightly regulated, the ability to specifically detect and characterize E. coli serotype O157:H7 in a sample, even in the presence of other E. coli serotypes, is useful.
It is desirable, therefore, to have a test for the accurate detection and characterization of E. coli O157:H7 in a sample.